HONG KONG — The spate of human bird flu cases in Vietnam this year suggests the deadly virus may be mutating in ways that are making it more capable of being passed between humans, according to a World Health Organization report.

The finding points to the greatest fear of health experts that the H5N1 virus could unleash a pandemic and kill millions around the globe if ever it gained the ability to be transmitted among humans efficiently.
While investigators could not prove human-to-human transmission had occurred, the report said, “the pattern of disease appeared to have changed in a manner consistent with this possibility.”

“They demonstrate that the viruses are continuing to evolve and pose a continuing and potentially growing pandemic threat,” the report said.

Klaus Stohr, WHO’s global influenza program coordinator, told a news briefing in Geneva: “We don’t know whether the pandemic will occur next week or next year...We should continue very intensively with pandemic preparations.”
H5N1 made its first known jump to humans in Hong Kong in 1997 and experts have always established that the mode of transmission was through direct contact with birds.

But the virus has mutated since, raising fears among experts that it may one day adapt in humans and become easily passed between them, setting off a pandemic.

In the six-page report, produced after an expert meeting in Manila from May 6-7, the WHO said at least 92 adults and children in Vietnam, Thailand and Cambodia had become ill after being infected with H5N1 since late 2003, and 52 of them had died. Stohr said the toll had risen to 97 cases with 53 deaths.

More clusters of infections involving household members have occurred, opening the possibility that “person-to-person transmission” may have taken place, the WHO said.

Eight such clusters were observed in North Vietnam this year alone, with recent cases spanning over a longer period.
“What we are seeing so far is a slight increase in clusters which could indicate more transmission...” Stohr said.
There was “circumstantial evidence” for such transmission—which could not be proved—but based on belief that a person became sick after being exposed only to an infected person and not to a sick chicken or duck, according to the expert.

“Then you can talk about very strong evidence for human to human transmission. That has happened in three clusters, two in Vietnam and one in Thailand...” Stohr added.

Regional Variations

Recent cases raised the possibility that apart from being exposed to ill birds, the victims might have been infected through prolonged exposure to birds that did not appear sick but which were shedding the virus, or through contact with other sick people.

The report also said that the age range of people infected in Vietnam this year had widened to between less than a year old to 80 years old.

Equally worryingly, human H5N1 viruses isolated from Vietnam were genetically distinct from strains in 2004. Scientists also found that viruses from northern Vietnam and Thailand had begun to form a separate cluster from those isolated from South Vietnam and Cambodia.

“It is possible that the avian H5N1 viruses are becoming more infectious for people, facilitating infection in a greater number or range of people and resulting in more clusters,” it said.

“It is possible that avian H5N1 viruses are becoming more capable of human-to-human transmission.”